'data.frame':   468 obs. of  3 variables:
 $ date                : Date, format: "2019-04-11" "2019-04-12" "2019-04-13" ...
 $ Object                : chr  "Object1" "Object2" "Object3" "Object 4" ...
 $ daily_sales: int  1 257 178 177 255 240 231 214 193 174 ...

ggplot(df, aes(x=date,
               y=daily_sales,
               colour=Object,
               group=Object)) +
  geom_line() +
  geom_vline(aes(xintercept=imp.dates[1]),
            linetype=4,
            colour="black")

ggplot(df, aes(x=date,
                   y=daily_sales,
                   colour=Object,
                   group=Object)) +
      geom_line() +
      geom_vline(aes(xintercept=imp.dates),
                linetype=4,
                colour="black")

Error: Aesthetics must be either length 1 or the same as the data (40): xintercept